1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a polarizing plate for achieving an improved brightness with a reduced total-reflection region, and a liquid crystal display device using the polarizing plate.
2. Discussion of the Related Art
Generally, it can be said that a light beam is a combination of two linear polarized light components having planes of polarization orthogonal to each other. Here, the plane of polarization is a plane including a light advance direction and an electric field, and a light component, in which the plane of polarization is limited to a single plane, is referred to as a linear polarized light component or plane polarized light component. The present invention relates to a method for fabricating a synthetic resin made polarized film used to extract a linear polarized light component.
Representative means used to extract a linear polarized light component from a light beam includes a polarizing prism formed of birefringent crystals, such as crystallized quartz.
With the recent tendency of utilizing a liquid crystal display device over various industrial fields, a thin-film shaped polarizing plate for use in the liquid crystal display device is already commercialized.
A thin-film shaped polarizing plate is widely used in various image forming apparatuses using a liquid crystal display device, such as a watch, pocket-size electronic notebook, notebook computer, camcorder, etc., or in other fields requiring the polarization of light, such as sunglasses.
On the purpose of use, the polarizing plate is valuable, in particular, when it keeps a constant polarizing performance and has a thin thickness. Such a polarizing plate is a technology-intensive product requiring all properties of high light-transmissivity, ultraviolet-ray extinction ability, water-resistance, dimensional stability, wear-resistance, etc.
A currently commercialized thin film shaped polarizing plate is configured such that a polarizing poly-vinyl-alcohol (PVA) film, which is treated with an iodine or dichromatic organic dye, is used as a polarizing substrate film, and a cellulose triacetate (CTA) film, which has a stability in dimension or deformation and wear-resistance, in addition to high transparency, ultraviolet-ray extinction ability and durability, is used as a protective film for protecting the polarizing substrate film.
In the related art, to attach a polarizing plate, which consists of a polarizing PVA film and CTA films stacked on and underneath the polarizing PVA film, to a front or rear surface of a liquid crystal display device, an adhesive layer has to be formed over the CTA film.
Although a solvent adhesive, such as an oil-based or water-based acrylic resin, has been used in the past, the adhesive requires the evaporation of a solvent after completing an attachment operation. Therefore, it is essential to provide a drying chamber for evaporation and drying of the solvent.
Further, when it is impossible to directly apply an adhesive between a polarizing substrate film and a protective film, the adhesive first has to be applied to the protective film that positively requires a release layer, and thereafter, the protective film has to be bonded to the polarizing substrate film. Furthermore, the bonding operation has a problem, such as generation of air bubbles, etc., and the resulting thin film shaped polarizing plate may contain volatile components upon the incomplete drying of the adhesive. A polyester (hereinafter, referred to as polyethylene terephthalate (PET)) film is generally used as the protective film having a release layer.
That is, the CTA film is used as a protective layer for the polarizing PVA film, and the PET film having a release layer is used as a protective layer for the overall polarizing plate onto which an adhesive is applied. The PET film is removed when the polarizing plate is attached to a liquid crystal display device.
Liquid crystal displays (LCDs), which are used in personal computers, etc., have a rapid increase in demand, and recently, the use range of LCDs is increasing even in the filed of monitors.
A polarizing plate used in LCDs is fabricated, for example, in such a manner that a poly-vinyl-alcohol (hereinafter, referred to as “PVA”) film is subjected to a variety of treatments, for example, a dyeing process using a dichromatic iodine or dichromatic dye, a crosslinking process using boron, borax, or the like, and a one-axis stretching process, and thereafter, the PVA film is dried and attached to a protective film, such as a tri-acetyl-cellulose (hereinafter, referred to as “TAC”), etc.
The above dyeing, crosslinking, and stretching processes are not essentially performed separately, and may be performed simultaneously. In addition, the implementation order of these processes can be freely determined as occasion demands.
Of the related art LCD polarizing plates fabricated as described above, a polarizing plate (decomposer), which is designed to be attached to a color filter substrate, has an anti-glare (AG) layer for preventing the polarizing plate from causing a glare phenomenon by an external light.
In the course of fabricating the polarizing film and the protective film of the polarizing plate, the AG layer serves to regulate the characteristics of external and internal lights, so as to provide a liquid crystal display device with an improved image quality.
FIG. 1 is a schematic exploded perspective view illustrating a related art liquid crystal display device.
As shown in FIG. 1, the related art liquid crystal display device comprises a liquid crystal display panel 30 for displaying an image, a fluorescent lamp 31 for emitting a light beam, and a U-shaped lamp housing 32 surrounding the fluorescent lamp 31. The liquid crystal display device further comprises a protective sheet 38, a first prism sheet 37, a second prism sheet 36, a diffusive plate 35, a light guide plate 33, and a reflective plate 34, which are attached to a surface of the liquid crystal display panel 30 in sequence.
In addition, a main support 39 is provided to receive and secure the liquid crystal display panel 30 and a backlight unit.
Here, the fluorescent lamp 31, lamp housing 32, diffusive plate 35, first prism sheet 37, second prism sheet 36, protective sheet 38, light guide plate 33, and reflective plate 34 constitute the backlight unit.
The backlight unit serves to irradiate a light beam onto a display region A of the liquid crystal display panel 30. Although not shown in the drawing, the display region A of the liquid crystal display panel 30 consists of two transparent substrates, each having a polarizing plate attached to an outer surface thereof, and liquid crystals injected between inner surfaces of the two transparent substrates.
The liquid crystal display device further comprises a drive circuit 40 for driving the display region A.
The backlight unit is operated in the following method. If the fluorescent lamp 31, installed to an end surface of the light guide plate 33, is turned on, a light beam, emitted from the fluorescent lamp 31, is reflected by the lamp housing 32. The reflected beam is transmitted to another end surface of the light guide plate 33, where no fluorescent lamp 31 is installed, through a cross section of the light guide plate 33. In this way, the light beam is spread over the surface of the light guide plate 33, and subsequently, is irradiated to the display region A of the liquid crystal display panel 30 by the diffusive plate 35.
In the above described liquid crystal display device, thin film transistors formed in the liquid crystal display panel 30 control pixels according to a signal from the drive circuit 40 such that the pixels selectively pass a light beam irradiated onto the display region A. With collection of the pixels selectively passing a light beam therethrough, an image is displayed on the display region of the liquid crystal display panel 30.
FIG. 2 is a view schematically illustrating a related art polarizing plate.
Generally, a liquid crystal display device is configured such that an array substrate having a matrix array of a plurality of pixel electrodes and a color filter substrate having Red, Green, and Blue color filters are bonded to each other. The liquid crystal display device is designed to form an image in such a manner that a reflected light beam of an external light beam (in the case of a reflective liquid crystal display device) or an internal light beam is polarized by a polarizing plate, and thereafter, the polarized light beam passes through liquid crystal molecules that are twisted by an electric field.
The polarizing plate is attached to either outer surface of both the array substrate and the color filter substrate. The polarizing plate (polarizer), attached to the outer surface of the array substrate, is used to polarize an internal light beam, which is emitted from a backlight unit and introduced into the polarizing plate, into a predetermined direction, and the polarized light beam passes through another polarizing plate (decomposer) attached to the outer surface of the color filter substrate after passing through liquid crystals.
Although the above decomposer and the polarizer are the same polarizing plates as each other in view of the fact that both of them are used to polarize a light beam, they are designated with different names on the basis of the characteristics of polarized light beams.
As shown in FIG. 2, the polarizing plate, which is attached to the outer surface of the color filter substrate of the liquid crystal display device, includes a polarizing film 50, a protective film 51 formed at a lower surface of the polarizing film 50, and an anti-glare (AG) layer formed at an upper surface of the polarizing film 50 for preventing the projection of an external image and the generation of a glare phenomenon.
An adhesive layer 54 is formed at a lower surface of the protective layer 51.
Here, the AG layer, which is formed on the polarizing film 50, is formed of an acrylate layer 53 containing SiO2-based polymer balls 52 mixed therein. A surface of the acrylate layer 53 has evenly formed waves due to the existence of the SiO2-based polymer balls 52.
The reason why the AG layer is formed on the surface of the polarizing plate is that the surface of the polarizing plate is an even surface and therefore, when an external light beam is irradiated to the even surface in a certain direction, the light beam is reflected at a specific angle, thereby causing a glare phenomenon at a specific viewing angle.
In the present invention, since the AG layer having the evenly formed surface waves is formed by mixing the polymer balls 52 into the acrylate layer 53, the polarizing plate can allow a light beam, irradiated from the outside, to be diffused at the surface of the polarizing plate by virtue of the waves formed on the surface of the AG layer, thereby achieving an anti-glare effect.
FIG. 3 is a view illustrating any possible problem of the polarizing plate shown in FIG. 2.
Specifically, FIG. 3 illustrates an acrylate layer to be formed on an upper surface of a polarizing film. As shown in FIG. 3, the acrylate layer 53 containing the polymer balls 52 mixed therein may be formed, at the upper surface thereof, with semi-spherical protrusions having different heights from each other.
The above polarizing plate of the related art, however, has the following problem.
When the acrylate layer 53 has surface waves due to the existence of the polymer balls 52 mixed therein, the waves tend to cause total-reflection or refraction of a light beam because of their different heights. This results in a great brightness difference between different regions of the polarizing plate. Further, such a brightness difference causes the surface of the polarizing plate to glitter, thereby resulting in a serious fatigue to the viewer's eyes.